Epoxy resins rank among the most popular types of adhesives, especially for use in metal-to-metal joining applications. In these applications, typified by the aircraft industries, epoxy resin adhesives offer a number of advantages over the alternative materials used for joining, e.g., ease of application, low temperature and pressure for curing, and high strength.
When two materials are bonded, the resultant composite has, in general, five elements: adherent No. 1/interface/adhesive/interface/adherent No. 2. The strength of the adhesive joint will be the strength of its weaker member. With stronger substrates, the failure will be either adhesive at an interface or cohesive within the glue. Failure will not be at the interface if the adherend surface has been properly prepared and the adhesive wets the adherend.
For many metals, the adhesion between the metal and epoxy adhesive is strong and durable. But it has been noted that difficulty is sometimes experienced during extended aging under atmospheric conditions. Oxidative degradation of the adhesive is initiated by several of the metals. Results of many years of experience and testing has generally shown that copper gives the poorest durability among the common metals. Copper-epoxy adhesion is important for many industrial applications, as for example, the joining of copper foil to fiberglass mat with an epoxy adhesive. In contrast to the good bond durability of aluminum and silver, the adhesive bond to copper fails rapidly in water or in highly humid environments. This poor durability of copper-epoxy adhesion is generally attributable to the unique properties of the copper oxides. In order to promote strong and durable bonding between the copper and the epoxy adhesive, several surface treatments have been introduced. One method utilizes a method wherein the vapor degreased copper surface is etched with a solution of sodium chlorite. This method is not suitable because the joint fails easily under shear stress and is unsuitable on thin copper films because of the high relative dimensional change caused by the etching. Another method utilizes a treatment with aqueous alkaline permanganate solution. Both of these methods fail to improve the water resistance of the copper-epoxy adhesive bond.
There is therefore adequate incentive to find a method, preferably a pretreatment method, for improving the bond strength of the copper-epoxy adhesive bond, particularly in moist environments.